by A. Santero, D. Cucci, E. Cattaneo,

F. Prosperi, M. PlacidiSTARA GLASS S.P.A.

NTRODUCTIONAll glass manufacturers

know that it is impossible totransfer all waste gas heat to

combustion air. In fact, it is well-known that waste gas has a highermass and higher specific heat thanair; even in a modern and performing regenerative furnace,at least 20-25 per cent of heat generated by fuel actually leavesthe furnace with the 450-500°Cwaste gas.

Stara Glass is, today, deeplycommitted in studying techniquesto exploit this otherwise lost heat.The Centauro system enables tosave it as a hot clean air flow, but afurther energetic advantage isoffered by cogeneration.

Stara Glass approached the prob-lem of cogeneration by using aninnovative external combustionengine, designed by Mr. Placidi, ahighly experienced mechanicalengineer, working in the glass andenergy industries.

The resulting and innovativeTrue_Tree system lowers the wastegas temperature down to 200°C,recovering thermal energy andtransforming a quota of 22-29 percent into mechanical and then elec-tric energy. The remaining energyis recovered as 100°C clean air.

Electrical energy thus generated

can be directly connected to thelocal network and it stands as animmediate economic saving, whichmust be added to eventual relativeenvironmental incentives.

Hot air, depending on specificcases, can be used to warm up workareas, transformed into warm waterfor sanitary uses, or be consideredas the first stage of a furnace heatrecovery system, thus increasingfurnace efficiency by 0.5-1.5 percent.

Recovering waste heat in thisway enables to save about 10-15per cent of melting costs.

The True_Tree engine has beeninvented for solar and biomass

applications, and has been adaptedby glass technicians in order to pro-vide an extremely simple, reliableand cost effective device, with apayback time of less than twoyears, without any impact on pro-duction lines.

At present, True_Tree is theonly cogeneration system specifi-cally designed for glass furnaceapplications.

PROJECT GUIDELINESFeatures and industrial strengths

of the project:- the engine creates an open cycle

with ambient air;- valveless two-stroke engine;

Working for over sixty years in the glass

industry, Stara Glass concentrates on

research and development. The

company’s most recent studies regard

methods to recover waste heat – as always

a big problem for glassmakers. The results of

these studies can be seen in this article,

where Stara has created a system to use this

heat, by means of an innovative external

combustion engine.

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True_Tree:

A COST EFFECTIVECOGENERATION IN

- crank connecting rod balancedmulti-cylinder engine;

- crankshaft and crank connectingrod system are oil lubricated;

- waste gas heat exchanger withmicro-vibration.Moreover, compared to a Stirling

engine, this open cycle eliminatessize and cost of the cold heatexchanger. Furthermore, Stirlingdynamic motions are hard to bal-ance, while in this engine all pis-tons work on the same shaft, result-ing in being easy to balance accord-ing to most common techniques.

Compared to a Rankine cycle,an air engine eliminates the costand size of the condenser. An air

engine, such as a compressor, doesnot need specialized personnel forcontrol and operation. Oil lubrica-tion is a reliable and cost effectivetechnology that prolongs the lifeof the device.

The simplicity of the machineenables to limit maintenance, whilethe vibrating exchanger preventsthe formation of dust layers on thewaste gas side. The project can beengineered with common commer-cial parts.

Air leaves the system at about100°C.

The engine is placed downstreamfrom the furnace, avoiding interfer-ence with furnace operations.

THE SYSTEMThe heart of the system is repre-

sented by an innovative exchangerthat allows to couple a hugeexchanging surface to an open cycleexothermic engine, which is modu-lar, valveless, and works at a lowrotation regime, about 500 rpm. Thecycle is not a Stirling, even if it isquite similar. In the present system,circuits are not pressurized, andthere is no special gas or transfer toanother cylinder, because the sec-ond cylinder is substituted by a fea-ture of the thermal exchanger. Thelow temperature and pressure levelsmake the system identifiable as reli-able and safe.

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SOLUTION FOR THE GLASS INDUSTRY

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Operating cycle:- piston ascent: compression of

cylinder air;- top dead centre: when pressure

reaches a chosen value, themobile exchanger enables air tocome into contact with exchang-ing surfaces;

- piston descent: the quick warm-up causes an expansion of theair, which pushes the pistondown;

- bottom dead centre: when lightsare open, the cleaning air expelshot air and fills the cylinderswith cold air.True_Tree represents the most

compact, cost effective and efficientsystem for thermal cogeneration.

The thermal exchanger is madeof easily interchangeable modules,which are protected from coarsedust by proper means.

The submicron fine particlescross the exchanger, withexchanging surfaces constantlyvibrating at ~7 Hz, thus prevent-ing the dust from depositing.Surface temperature and fluidvelocity also prevent the dust fromdepositing. If needed, a mechani-cal cleaning system can beinstalled.

The device is completely ther-mally insulated and waste gas

never comes into contact withcold elements (less than 200°C).

ELECTRIC ENERGYPRODUCTION

The True_Tree shaft is connect-ed to a three-phase electric motorthat can perform start-up and workas a power generator.

Once the engine is started-up,the generated power, directly or bymeans of an inverter, supplies thenet with the proper characteristics.

If the user decides to utilize thispower for the furnace (i.e. as apower booster), a group of con-densers can supply the necessaryreactive current.

To start-up the device, it is neces-sary to let waste gas cross the heatexchanger until it reaches the correcttemperature value. After this, theelectric engine starts the True_Treethat, automatically, stops absorbingpower and starts cogeneration. Thedevice includes an automatic systemof rotation speed control and anemergency system to stop operationin case of emergency.

SYSTEM FLEXIBILITYDepending on construction type,

the engine is designed in moduleswith one or two cylinders that canbe installed in series by means of

phasing keys, and therefore fits anyplant size.

Waste gas flow, rotation regimeand waste gas temperature can beregulated. This means thatTrue_Tree represents an alternativeto all waste gas temperature reduc-tion systems that are commonlyplaced before dust abatementplants. True-Tree therefore pro-vides an important opportunity toachieve an economic advantage.

This technology can be coupledwith Stara Glass’ Centauro and, asshown in the next pages, it is alsoadaptable to plants that alreadyinclude convective heat exchangers.

Each installation will be customized basing on customers’real needs, and considering fur-nace type, plant needs, availablespace, etc.

APPLICABILITY IN THE GLASSINDUSTRY

The glass industry is alwaysinterested in thermal recovery. Theavailability of an amount of hotwaste gas (450-550°C for regenera-tive and 800-900°C for recupera-tive furnaces) can guarantee highelectric generation efficiencies.

The energy required to melt 1ton of container glass is evaluatedas 1-1.5 MWh [ref: BAT Glass].

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Therefore the estimated energyamount that is recoverable from theproduction of 1 ton of glass resultsabout 30-45 kWh.

ModulesForeseen modules are compact

elements including:- waste gas inlet;- waste gas outlet;- hot air outlet;- connection to electric network;- Instrument air supply connec-

tion.All requirements for installation

consist in a waste gas line bypassand a relative extraction fan. If a fil-

This analysis regards an End Port furnace where waste gas temperature must be lowereddown to 200°C, the integrated solution allows to save all the relative heat, lowering wastegas temperature without any dilution. Twenty per cent of waste gas heat is transformed into electric energy, 62 per centinto hot clean air. Part of the clean air is sent to furnace regenerators with two advan-tages: combustion air temperature increases by about 50°C and regenerator effi-ciency increases by 1.89 per cent. The remaining hot air produced can be used directly for the warming of workplacesduring winter and/or to provide services with water by an air/water heat exchanger.This solution enables to save 5.5 per cent of furnace fuel and to produce 75.4 per centof electric power for boosting.

End Port or Side Port with bag filter or Mini-Centauro

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This solution enables to recover waste gas heat by lowering its temperature down to 400°C,the common value for the usage of an electrostatic filter. With this method, 26.3 per centof heat is turned into electric energy and 60.8 per cent into hot clean air.Part of the clean air is sent to furnace regenerators with two advantages: combustionair temperature increases by about 50°C and regenerator efficiency increases by1.89 per cent. Computing prudentially foresees a 0.95 per cent of furnace energysaving.The remaining hot air produced can be used directly for the warming of workplacesduring winter and/or to provide services with water by an air/water heat exchanger.This solution enables to save 1.4 per cent of furnace fuel and to produce 42.5 per centof electric power for boosting.

End Port or Mini-Centauro furnace with electrostatic filter

This analysis regards an integrated solution where waste gas temperature is lowered down to 200°C. The integra-ted solution enables to save all the relative heat, lowering waste gas temperature without any dilution.The True_Tree engine expels waste gas at 200°C and air at 100°C, producing about 450 kW of electric energy. Thehot air produced can be used directly for the warming of workplaces during winter and/or to provide services withwater by an air/water heat exchanger. This solution enables to save 5.8 per cent of furnace fuel and to produce53.3 per cent of electric power for boosting.

This simulation considers a plant already provided with a convective heat exchanger to lowerwaste gas temperature without any dilution.The True_Tree engine is regulated in order to provide the convective heat exchangerwith the necessary amount of heat to warm furnace combustion air up to 180°C, withtwo benefits: it increases outlet waste gas temperature by 100°C and improves rege-nerator efficiency by 4.10 per cent. Computing prudentially foresees a 2.05 per cent offurnace energy saving. The remaining hot air produced can be used directly for thewarming of workplaces during winter and/or to provide services with water by anair/water heat exchanger. This solution enables to save 6.4 per cent of furnace fuel and toproduce 49.8 per cent of electric power for boosting.

End Port or Mini-Centauro furnace with convective heat exchanger

Unit Melter furnace with bag filter

tering system is present, its fan issufficient to let the system work,thus eliminating the need of a wastegas temperature abatement system.

Examples of possible installationAll simulation has been carried

out considering the need of roomwarming for five months a year anda usage of 10 per cent of availablehot water. Computing of economicbenefits in Italy has been made withthe advice of Consulet ServiziS.r.l., Savona.

BENEFITSThe economic benefits of the

system are different in differentcountries, depending on energy

costs and on economic incentivesfor energy saving and electric ener-gy self-production.

For example, for every 1,000Sm3/h of 550°C waste gas, 22.5kWh are produced (=197,100 kWyear = 49.28 toe), expelling wastegas at 300°C, while the amount ofheat that is recoverable as hot airresults about 26 toe.

The chart on the next page showsthe value of kW produced with1000 Sm3/h of waste gas, enteringTrue_Tree at a temperature (theparameter in the chart) between450 and 700°C, and being expelledat 200-400°C.

Decreasing waste gas averagetemperature in the heat exchanger,

the efficiency of the system isdecreasing too, as can be seen inthe chart. In any case, at a lowwaste gas temperature, high pro-duction of energy can be reachedby using several modules.

Obviously, the higher the effi-ciency, the more advantageous theinvestment.

Each installation must thereforebe studied for the relative plant, inorder to achieve the highest bene-fits, with the lowest investment.

Savings deriving from electricenergy production are direct, and equal to produced energycosts. Saving deriving from white certificates must be added.Hot air can be used in many ways,

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This integrated solution enables to recover waste gas heat by lowering its tempe-rature to 400°C, common value for the usage of an electrostatic filter withoutdilution.The True_Tree engine works at its best efficiency, expelling waste gas at400°C, air at 100°C, and producing about 300 kW of electric energy. Thehot air produced can be used directly for the warming of workplacesduring winter and/or to provide services with water by an air/water heatexchanger. This solution enables to save 4.5 per cent of furnace fuel andto produce 37 per cent of electric power for boosting.

Unit Melter furnace with electrostatic filter

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such as heating of the glass mix,drying of the sand, hot water pro-duction, but the simplest and mosteconomical solution consists inroom heating and the productionof warm water.

As an example, in Italy, 1,000

Sm3/h of waste gas can pay backbetween EUR 32,000 and 105,000in the first five years, and betweenEUR 20,000 and 65,000 in thenext years.

The difference of resultsdepends on the needs of the differ-

ent applications. The data StaraGlass technicians need to simulatea possible custom solution are:- waste gas flow and temperature,

considering to minimize infiltra-tions;

- waste gas filtering temperature;

True_Tree can be coupled with Centauro, increasing the benefits of this already highly performing system. This solu-tion allows Centauro to save a further 2.2 per cent of furnace fuel, producing 18.1 per cent of boosting power.

Centauro Furnace

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- cost of used fuel and lowercalorific value;

- cost of electric energy;- value of white certificates or

other energy saving incentives;- fuel consumption;- utilized boosting power.

Furthermore, since the systemworks only with waste gas, it canguarantee furnace operations dur-ing a blackout, substituting a conti-nuity group.

OTHER POSSIBLEAPPLICATIONS

True_Tree can easily be adaptedto any system with heat sources thatare warmer than 400°C, as in thefields of solar energy, biomasses,and cement and steel production.

PROTOTYPEThe first 1 kW True_Tree proto-

type is undergoing experimentationand testing right now, and once itsefficiency has been proved, StaraGlass foresees the construction of a200-300 kW industrial prototype,

and then to start production of thereal scale engine during 2013.

RESEARCH ANDDEVELOPMENT

Stara Glass has been believingand investing in research and deve-lopment in the glass industry formore than ten years.

The number of Stara Glass’ filedpatents in spite of the present mar-ket crisis testifies the remarkablecommitment of people and meansfor engineering sustainable solu-tions, in order to reduce glass fur-naces consumption and pollutions.

In addition to True_Tree, StaraGlass is also committed in otherresearch projects:- advanced CFD studies on glass

furnaces;- advanced techniques for NOx

abatement;- cullet and glass mix preheating.

With True_Tree we are con-vinced that we have found a veryinteresting product both from atechnical and an economic point ofview.

All data of this article comefrom different experiences withscale models and mathematicalsimulations.

Those who are interested in dis-covering the evolution of the firstcogeneration engine especiallydesigned for the glass industry, areinvited to come and meet StaraGlass in Düsseldorf during theglasstec exhibition (22-26October 2012, Hall 13, StandA47), where the company’s tech-nicians will be present to demon-strate the latest developments ofthe project, to evaluate possiblesolutions and formulate estimatesand forecasts of economic returnon investment. ■

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STARA GLASS - HYDRA GROUPP.zza Rossetti 3 A/1 - 16129 Genova (GE) - ItalyTel: +39 – 010 – 564763E-mail: stara.green@hydragroup.it

www.staraglass.com